1. Field of the Invention
The present invention relates to a thermal fixing device and an image forming apparatus including the thermal fixing device.
2. Description of the Related Art
An image forming apparatus such as a laser printer is generally provided with a thermal fixing device including a heat roller and a pressure roller, and a toner transferred onto a sheet is thermally fixed during a period when the sheet passes through between the heat roller and the pressure roller.
In such a thermal fixing device, there is known one in which in order to increase a contact area between a heat roller and a sheet and to achieve quick and certain fixation, plural pressure rollers are provided in a conveyance direction of the sheet.
However, when the plural pressure rollers are provided, as the contact area of the sheet with the heat roller is increased, a curved portion along the curvature of the heat roller is increased. Thus, there is a disadvantage that for example, in the case where an envelope formed of a double paper or the like is fixed, a shift in the amount of conveyance occurs between its front sheet coming in contact with the heat roller and its back sheet coming in contact with the pressure roller, and wrinkles are apt to occur.
Thus, for example, JP-A-5-006118 proposes that a nip width of each of pressure rollers to a fixing roller is made 2.5 mm or less to prevent wrinkles from occurring when an envelope is fixed.